Universe is everything that exists, it is the spacetime and everything in it, all matter and energy as well as all the laws of nature by which it behaves. The fundamental natural laws of the Universe are researched by physics. The size of the whole universe is not known, it is thought to be infinite, however the size of the observable universe -- the part of it with which we can ever interact due to limited speed of light combined with constant expansion of space -- is about 93 billion light years in diameter, and contains an estimated number of 100 billion galaxies, each containing hundreds of billions of stars. Current science says the universe is about 13.7 billion years old and that it started with the Big Bang, a point in time from which everything began to rapidly expand from a single point in space.
Computers can be used to simulate certain parts of the universe, in fact all programs mimic the universe in some kind of simplified way, be it scientific simulations of planet collisions, government databases or games -- they all more or less accurately model the reality. We can also see computers as a way of creating smaller universes, which leads many to think our universe may itself be a simulation running on some computer in a "bigger" universe (note that this probably isn't testable and the debate isn't scientific, but we can lead philosophical discussions about it).
Many a question about the Universe are not only unknown, but unknowable altogether. These pertain to metaphysics rather than physics. We must accept that science and logic themselves have known theoretical limits, we KNOW it is impossible to obtain answers to some specific questions -- some have been mathematically proven to be unanswerable. On top of this our own brains (and computers, their helpers) are further limited by their size, available time and energy, so that even many of the answers that could be found cannot be comprehended by mankind simply because they may require galaxy-sized brains and billions of years to understand. Among these questions are for example whether the WHOLE Universe is truly infinite (and if so, then in what sense and in which dimensions), whether it is deterministic or WHY does it exist in the first place. Is the Universe a "subuniverse" (a "simulation") in yet a larger universe? Are there "parallel universes"? We can only speculate and form beliefs, and perhaps the most natural are the simplest answers: we can believe that the whole Universe (beyond the observable boundary) is infinite in both space and time AND likewise both towards large and small scales, just like real numbers can be infinitely multiplied but also infinitely divided. The boundary of observable universe presents the large-scale boundary of our knowability and quantum physics phenomena are the small scale boundary which manifests by perceived randomness. I.e. there are likely other infinite deterministic universes inside subatomic particles, whose behavior decides what we perceive as random outcomes (e.g. radioactive decay) on our scale, but we cannot see those universes. In this sense we can also posit that there are, indeed, parallel Universes: those that are separated from us by information boundaries such as the observable universe boundary. TODO: tired.
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